Screening experiments with antisera to human blood cell nuclear materials revealed a specific distribution of a few nuclear antigens in human cell lines representative of progressive stages of blood cell development. One prominent antigen of Mr = 55,000 was present only in one cell line, HL-60, representing the latest stages of myeloid cell development. Although this activity was easily detected using immunochemistry combined with the protein blotting technique, it is present in only small amounts in expressing cells as determined through conventional protein staining methods. Use of monospecific antibodies showed that the apparent specificity initially observed was not simply due to altered antigen electrophoretic migration characteristics in nonreacting cells. The nuclear location determined through the biochemical approach was confirmed by immunocytochemistry. Localization of the antigen in specimens of normal bone marrow, leukemic cell lines, and peripheral blood suggests that nuclei of many normal myeloid cells and peripheral blood monocytes contain this antigen. Only the cell lines capable of being induced to mature in vitro; expressed this antigen. In these studies, the investigation of cell type distribution will be extended to include the use of normal and pathologic blood and bone marrow specimens. The purpose will be to determine where in the differentiation schedule the expression of this antigen begins and to evaluate the usefulness of its presence in diagnosis. The solubility properties, the distribution in chromatin, binding properties, and the effects of microinjected antigen will be determined in an attempt to assign a possible function. These studies will be facilitated by the production and characterization of monoclonal antibodies. The detection of an Mr = 55,000 antigen in certain blood cells suggests its possible use as a marker in hematopathology and as a clue to understanding nuclear function. Specific mechanisms operating in the nucleus are believed to underlie the normal process of cellular differentiation. Abnormalities in differentiation and development have a role in a number of human diseases. The recognition of components involved in normal differentiation and the elucidation of their mechanisms of action will be an important step in providing the information necessary for prevention and intervention. (MI)